apt_preferences (5)

Leading comments

(The comments found at the beginning of the groff file "man5/apt_preferences.5".)

NAME

apt_preferences - Preference control file for APT

DESCRIPTION

The APT preferences file
/etc/apt/preferences
and the fragment files in the
/etc/apt/preferences.d/
folder can be used to control which versions of packages will be selected for installation.

Several versions of a package may be available for installation when the
sources.list(5)
file contains references to more than one distribution (for example,
stable
and
testing). APT assigns a priority to each version that is available. Subject to dependency constraints,
apt-get
selects the version with the highest priority for installation. The APT preferences override the priorities that APT assigns to package versions by default, thus giving the user control over which one is selected for installation.

Several instances of the same version of a package may be available when the
sources.list(5)
file contains references to more than one source. In this case
apt-get
downloads the instance listed earliest in the
sources.list(5)
file. The APT preferences do not affect the choice of instance, only the choice of version.

Preferences are a strong power in the hands of a system administrator but they can become also their biggest nightmare if used without care! APT will not question the preferences, so wrong settings can lead to uninstallable packages or wrong decisions while upgrading packages. Even more problems will arise if multiple distribution releases are mixed without a good understanding of the following paragraphs. Packages included in a specific release aren't tested in (and therefore don't always work as expected in) older or newer releases, or together with other packages from different releases. You have been warned.

Note that the files in the
/etc/apt/preferences.d
directory are parsed in alphanumeric ascending order and need to obey the following naming convention: The files have either no or "pref" as filename extension and only contain alphanumeric, hyphen (-), underscore (_) and period (.) characters. Otherwise APT will print a notice that it has ignored a file, unless that file matches a pattern in the
Dir::Ignore-Files-Silently
configuration list - in which case it will be silently ignored.

APT's Default Priority Assignments

If there is no preferences file or if there is no entry in the file that applies to a particular version then the priority assigned to that version is the priority of the distribution to which that version belongs. It is possible to single out a distribution, "the target release", which receives a higher priority than other distributions do by default. The target release can be set on the
apt-get
command line or in the APT configuration file
/etc/apt/apt.conf. Note that this has precedence over any general priority you set in the
/etc/apt/preferences
file described later, but not over specifically pinned packages. For example,

apt-get install -t testing some-package

APT::Default-Release "stable";

If the target release has been specified then APT uses the following algorithm to set the priorities of the versions of a package. Assign:

priority 1

to the versions coming from archives which in their
Release
files are marked as "NotAutomatic: yes" but
not
as "ButAutomaticUpgrades: yes" like the Debian
experimental
archive.

priority 100

to the version that is already installed (if any) and to the versions coming from archives which in their
Release
files are marked as "NotAutomatic: yes" and "ButAutomaticUpgrades: yes" like the Debian backports archive since
squeeze-backports.

priority 500

to the versions that do not belong to the target release.

priority 990

to the versions that belong to the target release.

The highest of those priorities whose description matches the version is assigned to the version.

If the target release has not been specified then APT simply assigns priority 100 to all installed package versions and priority 500 to all uninstalled package versions, except versions coming from archives which in their
Release
files are marked as "NotAutomatic: yes" - these versions get the priority 1 or priority 100 if it is additionally marked as "ButAutomaticUpgrades: yes".

APT then applies the following rules, listed in order of precedence, to determine which version of a package to install.

*
Never downgrade unless the priority of an available version exceeds 1000. ("Downgrading" is installing a less recent version of a package in place of a more recent version. Note that none of APT's default priorities exceeds 1000; such high priorities can only be set in the preferences file. Note also that downgrading a package can be risky.)

*
Install the highest priority version.

*
If two or more versions have the same priority, install the most recent one (that is, the one with the higher version number).

*
If two or more versions have the same priority and version number but either the packages differ in some of their metadata or the
--reinstall
option is given, install the uninstalled one.

In a typical situation, the installed version of a package (priority 100) is not as recent as one of the versions available from the sources listed in the
sources.list(5)
file (priority 500 or 990). Then the package will be upgraded when
apt-get install some-package
or
apt-get upgrade
is executed.

More rarely, the installed version of a package is
more
recent than any of the other available versions. The package will not be downgraded when
apt-get install some-package
or
apt-get upgrade
is executed.

Sometimes the installed version of a package is more recent than the version belonging to the target release, but not as recent as a version belonging to some other distribution. Such a package will indeed be upgraded when
apt-get install some-package
or
apt-get upgrade
is executed, because at least
one
of the available versions has a higher priority than the installed version.

The Effect of APT Preferences

The APT preferences file allows the system administrator to control the assignment of priorities. The file consists of one or more multi-line records separated by blank lines. Records can have one of two forms, a specific form and a general form.

*
The specific form assigns a priority (a "Pin-Priority") to one or more specified packages with a specified version or version range. For example, the following record assigns a high priority to all versions of the
perl
package whose version number begins with "5.10". Multiple packages can be separated by spaces.

Package: perl
Pin: version 5.10*
Pin-Priority: 1001

*
The general form assigns a priority to all of the package versions in a given distribution (that is, to all the versions of packages that are listed in a certain
Release
file) or to all of the package versions coming from a particular Internet site, as identified by the site's fully qualified domain name.

This general-form entry in the APT preferences file applies only to groups of packages. For example, the following record assigns a high priority to all package versions available from the local site.

Package: *
Pin: origin ""
Pin-Priority: 999

A note of caution: the keyword used here is "origin" which can be used to match a hostname. The following record will assign a high priority to all versions available from the server identified by the hostname "ftp: ftp.de.debian.org"

This should
not
be confused with the Origin of a distribution as specified in a
Release
file. What follows the "Origin:" tag in a
Release
file is not an Internet address but an author or vendor name, such as "Debian" or "Ximian".

The following record assigns a low priority to all package versions belonging to any distribution whose Archive name is "unstable".

Package: *
Pin: release a=unstable
Pin-Priority: 50

The following record assigns a high priority to all package versions belonging to any distribution whose Codename is "stretch".

Package: *
Pin: release n=stretch
Pin-Priority: 900

The following record assigns a high priority to all package versions belonging to any release whose Archive name is "stable" and whose release Version number is "8".

Package: *
Pin: release a=stable, v=8
Pin-Priority: 500

The effect of the comma operator is similar to an "and" in logic: All conditions must be satisfied for the pin to match. There is one exception: For any type of condition (such as two "a" conditions), only the last such condition is checked.

APT also supports pinning by
glob(7)
expressions, and regular expressions surrounded by slashes. For example, the following example assigns the priority 500 to all packages from experimental where the name starts with gnome (as a
glob(7)-like expression) or contains the word kde (as a POSIX extended regular expression surrounded by slashes).

Package: gnome* /kde/
Pin: release a=experimental
Pin-Priority: 500

The rule for those expressions is that they can occur anywhere where a string can occur. Thus, the following pin assigns the priority 990 to all packages from a release starting with trusty.

Package: *
Pin: release n=trusty*
Pin-Priority: 990

If a regular expression occurs in a
Package
field, the behavior is the same as if this regular expression were replaced with a list of all package names it matches. It is undecided whether this will change in the future; thus you should always list wild-card pins first, so later specific pins override it. The pattern "*" in a Package field is not considered a
glob(7)
expression in itself.

How APT Interprets Priorities

Priorities (P) assigned in the APT preferences file must be positive or negative integers. They are interpreted as follows (roughly speaking):

P >= 1000

causes a version to be installed even if this constitutes a downgrade of the package

990 <= P < 1000

causes a version to be installed even if it does not come from the target release, unless the installed version is more recent

500 <= P < 990

causes a version to be installed unless there is a version available belonging to the target release or the installed version is more recent

100 <= P < 500

causes a version to be installed unless there is a version available belonging to some other distribution or the installed version is more recent

0 < P < 100

causes a version to be installed only if there is no installed version of the package

P < 0

prevents the version from being installed

P = 0

has undefined behaviour, do not use it.

The first specific-form record matching an available package version determines the priority of the package version. Failing that, the priority of the package is defined as the maximum of all priorities defined by generic-form records matching the version. Records defined using patterns in the Pin field other than "*" are treated like specific-form records.

For example, suppose the APT preferences file contains the three records presented earlier:

*
The most recent available version of the
perl
package will be installed, so long as that version's version number begins with "5.10". If
any
5.10* version of
perl
is available and the installed version is 5.14*, then
perl
will be downgraded.

*
A version of any package other than
perl
that is available from the local system has priority over other versions, even versions belonging to the target release.

*
A version of a package whose origin is not the local system but some other site listed in
sources.list(5)
and which belongs to an
unstable
distribution is only installed if it is selected for installation and no version of the package is already installed.

Determination of Package Version and Distribution Properties

The locations listed in the
sources.list(5)
file should provide
Packages
and
Release
files to describe the packages available at that location.

The
Packages
file is normally found in the directory
.../dists/dist-name/component/arch: for example,
.../dists/stable/main/binary-i386/Packages. It consists of a series of multi-line records, one for each package available in that directory. Only two lines in each record are relevant for setting APT priorities:

the Package: line

gives the package name

the Version: line

gives the version number for the named package

The
Release
file is normally found in the directory
.../dists/dist-name: for example,
.../dists/stable/Release, or
.../dists/jessie/Release. It consists of a single multi-line record which applies to
all
of the packages in the directory tree below its parent. Unlike the
Packages
file, nearly all of the lines in a
Release
file are relevant for setting APT priorities:

the Archive: or Suite: line

names the archive to which all the packages in the directory tree belong. For example, the line "Archive: stable" or "Suite: stable" specifies that all of the packages in the directory tree below the parent of the
Release
file are in a
stable
archive. Specifying this value in the APT preferences file would require the line:

Pin: release a=stable

the Codename: line

names the codename to which all the packages in the directory tree belong. For example, the line "Codename: stretch" specifies that all of the packages in the directory tree below the parent of the
Release
file belong to a version named
stretch. Specifying this value in the APT preferences file would require the line:

Pin: release n=stretch

the Version: line

names the release version. For example, the packages in the tree might belong to Debian release version 8. Note that there is normally no version number for the
testing
and
unstable
distributions because they have not been released yet. Specifying this in the APT preferences file would require one of the following lines.

Pin: release v=8
Pin: release a=stable, v=8
Pin: release 8

the Component: line

names the licensing component associated with the packages in the directory tree of the
Release
file. For example, the line "Component: main" specifies that all the packages in the directory tree are from the
main
component, which entails that they are licensed under terms listed in the Debian Free Software Guidelines. Specifying this component in the APT preferences file would require the line:

Pin: release c=main

the Origin: line

names the originator of the packages in the directory tree of the
Release
file. Most commonly, this is
Debian. Specifying this origin in the APT preferences file would require the line:

Pin: release o=Debian

the Label: line

names the label of the packages in the directory tree of the
Release
file. Most commonly, this is
Debian. Specifying this label in the APT preferences file would require the line:

Pin: release l=Debian

All of the
Packages
and
Release
files retrieved from locations listed in the
sources.list(5)
file are stored in the directory
/var/lib/apt/lists, or in the file named by the variable
Dir::State::Lists
in the
apt.conf
file. For example, the file
debian.lcs.mit.edu_debian_dists_unstable_contrib_binary-i386_Release
contains the
Release
file retrieved from the site
debian.lcs.mit.edu
for
binary-i386
architecture files from the
contrib
component of the
unstable
distribution.

Optional Lines in an APT Preferences Record

Each record in the APT preferences file can optionally begin with one or more lines beginning with the word
Explanation:. This provides a place for comments.

EXAMPLES

Tracking Stable

The following APT preferences file will cause APT to assign a priority higher than the default (500) to all package versions belonging to a
stable
distribution and a prohibitively low priority to package versions belonging to other
Debian
distributions.

With a suitable
sources.list(5)
file and the above preferences file, any of the following commands will cause APT to upgrade to the latest
stable
version(s).

apt-get install package-name
apt-get upgrade
apt-get dist-upgrade

The following command will cause APT to upgrade the specified package to the latest version from the
testing
distribution; the package will not be upgraded again unless this command is given again.

apt-get install package/testing

Tracking Testing or Unstable

The following APT preferences file will cause APT to assign a high priority to package versions from the
testing
distribution, a lower priority to package versions from the
unstable
distribution, and a prohibitively low priority to package versions from other
Debian
distributions.

With a suitable
sources.list(5)
file and the above preferences file, any of the following commands will cause APT to upgrade to the latest
testing
version(s).

apt-get install package-name
apt-get upgrade
apt-get dist-upgrade

The following command will cause APT to upgrade the specified package to the latest version from the
unstable
distribution. Thereafter,
apt-get upgrade
will upgrade the package to the most recent
testing
version if that is more recent than the installed version, otherwise, to the most recent
unstable
version if that is more recent than the installed version.

apt-get install package/unstable

Tracking the evolution of a codename release

The following APT preferences file will cause APT to assign a priority higher than the default (500) to all package versions belonging to a specified codename of a distribution and a prohibitively low priority to package versions belonging to other
Debian
distributions, codenames and archives. Note that with this APT preference APT will follow the migration of a release from the archive
testing
to
stable
and later
oldstable. If you want to follow for example the progress in
testing
notwithstanding the codename changes you should use the example configurations above.

With a suitable
sources.list(5)
file and the above preferences file, any of the following commands will cause APT to upgrade to the latest version(s) in the release codenamed with
stretch.

apt-get install package-name
apt-get upgrade
apt-get dist-upgrade

The following command will cause APT to upgrade the specified package to the latest version from the
sid
distribution. Thereafter,
apt-get upgrade
will upgrade the package to the most recent
stretch
version if that is more recent than the installed version, otherwise, to the most recent
sid
version if that is more recent than the installed version.

apt-get install package/sid

FILES

/etc/apt/preferences

Version preferences file. This is where you would specify "pinning", i.e. a preference to get certain packages from a separate source or from a different version of a distribution. Configuration Item:
Dir::Etc::Preferences.

/etc/apt/preferences.d/

File fragments for the version preferences. Configuration Item:
Dir::Etc::PreferencesParts.